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In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside in the case of external fertilization. The fertilized egg cell is known as the zygote. [2] [5]
Some females abort or resorb their own young while they are still in development after a new male takes over; this is known as the Bruce effect. [31] This may prevent their young from being killed after birth, saving the mother wasted time and energy. However, this strategy also benefits the new male.
For most species, the amount a fetus grows before birth determines the length of the gestation period. Smaller species normally have a shorter gestation period than larger animals. [2] For example, a cat's gestation normally takes 58–65 days while an elephant's takes nearly 2 years (21 months). [3]
In viviparous animals (animals whose offspring spend at least some time developing within a parent's body), the offspring is typically referred to as an embryo while inside of the parent, and is no longer considered an embryo after birth or exit from the parent. However, the extent of development and growth accomplished while inside of an egg ...
Human babies are unique in the animal kingdom due to their large head size relative to their bodies. This has an effect on the birthing process for humans as the bipedal gait of a human causes the birthing canal to be relatively narrow and twisted in the middle.
Parthenogenesis is the growth and development of embryo or seed without fertilization. Parthenogenesis occurs naturally in some species, including lower plants (where it is called apomixis ), invertebrates (e.g. water fleas , aphids , some bees and parasitic wasps ), and vertebrates (e.g. some reptiles, [ 3 ] some fish, [ 4 ] and very rarely ...
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Due to the fact that placental mammals and marsupials nourish their developing embryos via the placenta, the ovum in these species does not contain significant amounts of yolk, and the yolk sac in the embryo is relatively small in size, in comparison with both the size of the embryo itself and the size of yolk sac in embryos of comparable developmental age from lower chordates.